The present invention relates to an improved method and apparatus for pumping media which are relatively thick or clotted or contain air, i.e. liquids or various kinds of suspensions. The method and apparatus in accordance with the present invention are especially suitable for pumping medium consistency (8 to 20%) or high consistency (over 20%) fiber suspensions of the pulp and paper industry. The method part of the invention especially relates to a method of eliminating or minimizing the disadvantages caused by air and/or gases in the medium or which are absorbed by the medium. The apparatus part of the present invention especially relates to the structure of an impeller for use in a centrifugal pump.
There are several known centrifugal pumps that have been used and are still being used in the wood processing industry for pumping fiber suspensions. The largest group is basically formed by conventional centrifugal pumps, which are somewhat modified by non-essential changes, to enable the pumping of pulp therewith. An example of this type of change is the installation of inducers in front of the actual impeller to facilitate the flow of pulp to the impeller of the pump itself. In spite of a number of attempts and minor structural changes it has not been possible to use the above described pumps for the pumping of pulp, having a consistency exceeding 6-8%. This is due to both an increase of the gas content of the pulp simultaneously with an increase of the consistency thereof, whereby the air or gas bubble accumulated at the center of the impeller prevents the pulp from effectively contacting the impeller as well as due to poor flow-ability of the high consistency pulp within the suction duct of the pump or from the pulp containing chamber to the suction duct of the pump.
The next stage in the late 1970s is governed by the so-called MC.sup.TM - pump (MC =medium consistency), which is characterized in that a rotor is mounted in the suction opening of the pump and extends through the suction duct to a short extent inside the mass tower, drop leg or the like. The rotor is used for loosening the bonds between fibers of the suspension by feeding energy in the form of a field of shear forces to the pulp, whereby the pulp flows more easily to the impeller of the pump. The object with these pumps was to achieve the ability to pump pulps having a consistency of 8 to 15%. The main problem encountered with this kind of medium consistency pulp appeared to be the poor flowability of the pulp in the suction duct of the pump and consequently, the invention was aimed at facilitating the pulp flow to the impeller within the flow duct. Various embodiments of such pumps are illustrated, for example, in the specifications of U.S. Pat. Nos. 4,410,337, 4,435,193 and 4,637,779. In these embodiments the pulp being pumped is both fluidized and the gas, mostly air, which is harmful in the further processing of the pulp and in the pumping thereof is discharged. The fluidization is carried out by rotor blades mounted inside a relatively long suction duct of the pump, the blades being substantially located in radial planes and extend axially, although in some embodiments helically wound rotor blades are also to some extent used. The accumulation of gas at the hollow center of the rotor takes place in all illustrated embodiments in front of the impeller due to centrifugal force, wherefrom the gas is further discharged through the openings in the rear plate of the impeller most commonly by means of the suction created by a vacuum pump.
Regarding the structural details of the MC-pumps in the prior art it is noted that the rotor in all embodiments extends to some extent into the pulp containing space. A detailed description thereof is provided in the most recent publication, U.S. 4,637,779, in which the rotor is described to be extending into said space by a distance of about 3 inches, in other words of about 75 mm. This distance is actually the maximum distance, as most of the production pumps include a rotor which does not extend that far into the pulp containing chamber. The maximum dimension is thus at most about one-half times the diameter of the suction duct, i.e. the ratio decreases in reality as the diameter of the suction duct increases. In practice the suction duct of even the smallest MC-pump has a diameter of about 150 mm resulting in the above relation. However, when the diameter of the suction duct is increased the actual distance of the extension of the rotor into the pulp chamber remains substantially the same.
Initial development of the MC-pumps was based on the assumption that the fundamental problem for pumping high consistency pulp was to overcome the friction present between the pulp and the wall of the suction duct. Accordingly attempts were made to eliminate the friction by fluidizing the pulp in the suction duct. A second problem appeared to be the discharge or transfer of pulp from the vacuum chamber or from the drop leg to the suction duct, because high consistency pulp tends to gradually fill openings surrounded by sharp edges, in other words, the suction opening of the pump. This theory resulted in the extension of the fluidizing rotor to reach to some extent to the inside of the pulp chamber to allow the rotor to remove the fibers and fiber particles stuck to the edges of the suction duct and to prevent the clogging of the suction opening. However, the prior art maintained the theory that the flow of the material being pumped should be as laminar as possible when reaching the pump so as to avoid losses in flow, e.g. pressure loss. Reference thereof may still be found, for example, in the above-mentioned U.S. patent specification 4,637,779, in which at column 2, lines 24-30 it is stated that an apparatus in accordance with the prior art technique generates in front of the suction opening of the pump and around it a "doughnut"-shaped, turbulent, at least partly fluidized, zone which is located in close proximity to the edges of the pump. In accordance with the former theory this U.S. patent specification comes to the conclusion that the described phenomenon disturbs the pumping and, therefore, the ends of the rotor blades extending into the mass tower or the like are bent so as to subject the pulp to a force component which is directed towards the suction opening of the pump. According to the specification this solution is based on the fact that it is possible to apply or exert pressure onto the inflowing pulp, thus also facilitating the discharge of gas from the front side of the impeller of the pump.
Because the disclosed pumps have never entered the market there is good reason to doubt the operation of the described apparatus as well as the exactness of the above-mentioned conclusions at least as they relate to the higher end of the 6 to 20% consistency range mentioned in the specification. In fact, it is believed that in the embodiment shown in said U.S. patent a hollow arch-like space is readily formed in the pulp at higher pulp consistencies in front of the ends of the rotor blades because the intent is especially to prevent the circulation of the pulp in the pulp vessel. In other words, the pulp is drawn as discretely as possible directly from the vessel to the pump. This problem is, however, encountered only with pulp having a consistency of 10 to 15% depending on the physical and chemical qualities of the pulp.
Accordingly, when pumping pulp with MC-pumps and even at a consistency lower than medium consistency and although the pump and rotor are able to efficiently treat the pulp in the suction duct and immediate vicinity thereof, the problem appears to be based on the poor transfer of pulp from the mass tower or the like to the suction duct of the pump. The bases of this problem are two fold: first, the arching of pulp in the pulp space, in other words, the formation of an empty arch-like space in front of the suction opening of the pump and, second, the friction between the pulp and the walls of said space which slows down the downward flow of the pulp.
It has been noted in the experiments performed that an especially efficient method of preventing the arching of the pulp and reducing the friction between the pulp and the wall of the pulp space is to impart a circulating movement to the pulp in the pulp vessel. This kind of circulating movement may generate enough turbulence in the pulp layer close to the wall of the pulp space so that small pulp particles are generated when larger pulp particles are broken, and the small particles act in a way as bearing balls between the pulp and the wall, whereby the friction between the pulp and the vessel wall is decreased and the pulp will flow downwards faster and easier.
When pumping experiments have been performed with an MC-pump in accordance with the prior art simulating the mill conditions it has been noted that gas will flow through the lumpy pulp in the pulp space to the suction opening of the pump, which has, as already mentioned above, a "doughnut-shaped" fluidized ring along the edges thereof and which also has, in a certain way, an open center part, whereby the lumpy pulp may be directly subjected to the suction of the pump and the gas discharge system thereof and even to the air space in the upper part of the pulp containing space. Accordingly gas will flow both from the many spaces between the pulp lumps and, in the case of a very high consistency pulp also from the upper part of the pulp container, the air space, to the pump.
The reason this phenomenon has not been observed before is partly due to, on one hand, that only small amounts of air and, on the other hand, that pulp containing only small particles have been pumped in the tests with the pulp having a consistency not above 15%, whereby the air spaces between the pulp particles are small and they do not reach the surface of the pulp space. Said problem occurs only at the stage when the amount of free water (water not absorbed in the fibers) in the pulp is decreased to a point so low that it does not have time to filter to the bottom part of the mass tower or the like to form a layer of water and pulp lumps there. No definite consistency limit or the like may be given as to the appearance of said problem, because it depends on many factors such, for example, as the consistency of the pulp itself, the length of the fibers of the pulp, the speed at which the pulp flows downwards in the tower, etc. The problem, however, appears soon after the consistency of about 10% is exceeded.
When experiments simulating the mill conditions have been performed with the pump in accordance with the present invention, it has been noted that when the rotor of the pump has been extended far enough into the suction chamber and especially when the end of the rotor has been provided with foils intensifying the circulating flow heavily criticized, for example in U.S. Pat. specification No. 4,637,779, the amount of gas removed by the pump from the medium is substantially reduced.